1. Field of the Invention
The present invention relates to an optical disc on which signals are recorded and from which recorded signals are read by means of reflection or transmission of laser light.
2. Description of the Prior Art
At present, optical discs are commercially available as digital audio discs (compact disc) and optical video discs. They are read only discs on which unerasable information has been recorded. On the other hand, researches and developments have become active for write once optical discs and erasable optical discs which are expected to be promising storage media for images, documents and data.
The optical disc is made up of a substrate and a recording film formed thereon, and the substrate is made of glass or transparent resin. The resin substrate is required to have the following performances.
(1) Good moldability and good transferability of mold pattern.
(2) High light transmittance.
(3) Small optical anisotropy (Referred to as birefringence hereinafter).
(4) High mechanical strength.
(5) Good durability to retain the initial physical properties for a long time under severe environmental conditions.
(6) Good receptivity to the recording film.
In the case of the read only optical disc, the recording film is aluminum deposited on the disc substrate. In the case of the write once or erasable optical disc, there have been developed various types of recording films such as melt type, phase transition type, opto-magnetic type, and sublime type.
The optical disc substrate should be unreactive to the recording film. In other words, it should not contain a volatile substance or bleeding substance which reacts with the recording film to deteriorate it.
In view of the above-mentioned requirements, raw material of the optical disc substrate has been an acrylic resin or specific polycarbonate resin.
There is disclosed in Japanese Patent Application Laid-open Nos. 58-126119 and 58-180553 a specific polycarbonate resin for optical disc substrate which has a small average molecular weight and contains at least 0.005 wt % of antioxidant. The average molecular weight of the polycarbonate resin is made relatively small thereby reducing birefringence. According to these prior art references, the polycarbonate resin is incorporated with a phosphorus-based antioxidant to prevent the thermal degradation (thermal decomposition) which would otherwise occur at the time of molding. With the amount at least 0.005 wt %, usually 0.01 to 0.1 wt %, as disclosed, the added antioxidant adversely affects the recording film in a high-temperature, high-humidity environment.